Modern enterprises are often geographically dispersed, with servers, data storage facilities and other communications devices interconnected through a metropolitan or wide area network (MAN/WAN). Enterprises typically rely on three main types of MAN/WAN infrastructures:                a) IP/Ethernet networks;        b) Dense wavelength division multiplexing (DWDM) networks; and        c) Synchronous optical networks (SONET).        
Enterprises often require data transmission solutions that are able to support a variety of protocols, individually and concurrently. For example, a typical enterprise may need to transport Single Byte Command Code Sets Connection (SBCON) and Gigabit Ethernet (GbE) formatted data in support of particular applications. As depicted in FIG. 1, SONET/SDH network 2 connects multiplexers 4, 6 to SBCON and GbE to switches 8, 10 which in turn connect to client devices 12, 14, performing storage functions. The multiplexers/demultiplexers 4, 6 such as Akara Corporation's Optical Utility Services Platform (OUSP)™ multiplex/demultiplex client data protocols such as Fibre Channel (FC), Fibre Connection (FICON), Gigabit Ethernet (GbE), Enterprise System Connection (ESCON) and Single Byte Command Code Sets Connection (SBCON) directly into, for example, a SONET/SDH OC-3, OC-12, or OC-48 payload and transports it natively using SONET/SDH network 2 between geographically dispersed data centres A and B.
It will be understood by those in the art that frame based protocols such as Ethernet and SBCON, use idles between consecutive frames to both fill quiet time and guarantee a minimum time between frames. Such minimum inter-frame times may be required, for example, to ensure that the receiving device can process the current frame before subsequent frames are transmitted, in order to avoid receive buffer overflow resulting in data loss.
It will also be understood by those skilled in the art that some frame based protocols such as the American National Standards Institute (ANSI) standard interface protocol SBCON, use minimum spacing between frames which is negotiated between communicating pairs of end systems. This is referred to as time-based pacing. The pacing parameters are negotiated during initialization between the communicating devices. Devices used to transport multiple pairs of communicating devices across a single transport medium, such as a MAN/WAN link, may have to preserve different sets of pacing parameters for each communicating pair of end systems. As will be also appreciated by those in the art, time based pacing is one flow control technique associated with the data link layer defined in the Open Systems Interconnection (OSI) Reference Model developed by the International Organization for Standardization (ISO). A sublayer of the data link layer, the logical link control sublayer, is responsible for, among other things, controlling frame synchronization, error checking and flow control. Time based pacing is one of two primary flow control techniques, the other being credit-based flow control, whereby a data link regulates the flow of traffic by sending credit tokens to sources.
Either participating in, or snooping, the initialization procedure to determine potentially many different pacing parameters can be quite complicated and introduce undesirable system costs as well as additional transport latency. In addition, maintaining pacing information across transport media which transport information at rates different than the native protocol rate increases the complexity of the implementation.
At the present time, known solutions to this problem either terminate the protocol prior to the transport medium or provide a transport medium that is synchronous and transparent to the protocol. Devices which terminate the protocol are very complex and therefore expensive, since they must be application aware. Such implementations can require device spoofing and protocol translation. Further, processing capacity and cost places limits on the number of communicating devices that can be supported. Latency also increases due to the protocol interactions at both the entry and exit points of the transport systems.
Devices which provide synchronous protocol transparency such as DWDM, eliminate the complexities of protocol termination but they prevent transmission at transport rates other than those of the native protocol. This significantly reduces the connectivity options available.
There is a need, therefore, for a simple, low cost and reliable method for preserving data link layer (i.e. flow control) pacing information between clients communicating natively at rates other than the transfer rate of the transport medium.